This invention relates generally to data transmission systems and particularly to a method of operating a high speed data transmission system that is capable of error-free transfer of data in a noisy medium or in one containing other aberrations. The inventive method will be of particular benefit when used in a cellular telephone environment.
The prior art discloses many information transmission systems. A fundamental form is a basic telephone to convey voice frequency electrical signals over a pair of wires. Information that is transmitted over telephone lines is generally less susceptible to noise, phase changes and interruptions, than is information that is sent over a radio frequency link. Dedicated or so-called "leased lines" are telephone lines that essentially bypass the telephone switching network and are even less susceptible to noise. Accurate, high speed transfer of data in such a medium is quite realizable.
The fairly recent cellular telephone service uses radio links to interconnect mobile telephones with other mobile telephones and with conventional telephones via a telephone switching network. Mobile telephone service involves a very hostile environment. It uses frequency modulation of an RF carrier that is susceptible to interference, noise, fading, signal loss and phase shifting. While presently analog voice signals are carried reasonably well, the analog transmission of digital data, which is readily carried out over normal telephone lines, is rendered nearly impossible due to noise and other aberrations, such as those above mentioned, in the cellular telephone medium.
There are two fundamental problems in data transmission: accuracy and speed. Conventional data transmission systems are known wherein data is formulated into blocks or packets and some form of error protection, such as a check sum or a cyclic redundancy code (CRC), is appended to the data for providing a means at the receiver to determine whether the data has been corrupted or impaired during transmission. If an impaired packet is received, the receiver signals the transmitter to retransmit, beginning with the impaired packet.
There are well-known mathematical and physical techniques for compressing data to reduce the necessity of transmitting repetitive bit groupings. In data compression, a "compression table") (or "hash" table as it is colloquially referred to) is constructed of patterns of characters or bit groupings with the various patterns being identified. The pattern identifiers are transmitted rather than the character patterns or bit groupings themselves. These techniques can be shown to improve system throughput by allowing more data to be sent in a shorter time period and are in common usage. In the preferred embodiment, a system of data compression based upon the Lempel-Zev-Welch (LZW) compression technique is utilized. Information on LZW compression may be found in A Method for Construction of Minimum Redundancy Codes by D. A. Huffman, Proceeding of the IRE, 1952 and Pattern Matching in Strings SIAM, by D. E. Knuth, Journal on Computing, June 1977. There are many other treatises on the subject.
The technique known as forward error correction (FEC) is also known for detecting and correcting errors in data. Mathematical models by Huffman and Golay, (and others), for error correcting bit strings are known. In all of the FEC systems, the number of bytes of transmitted information is significantly increased and generally permits the detection of only a limited number of bit errors in a protected bit packet and the correction of a lesser number of bit errors in the protected bit packet. Errors in excess of the correctable number require retransmission. FEC systems have been sparingly used.
In Golay encoding, which is used in the invention, a check word is created to forward error correct a code word. A code word is generally the same as a data word. The term, however, is used herein to differentiate from a data word since, in the embodiment of the invention chosen for purposes of description, a 12 bit code word is equal to one and one-half 8 bit data words. A significant drawback to Golay FEC is that the size of the check word is non-linearly related to the number of bits to be forward error corrected. For example, in a system where three bits are protected, the check word size is equal to the code word size, i.e., the number of bits processed is doubled. Throughput suffers accordingly. FEC encoding in a high speed transmission system, such as one operating at 4800 Baud, is of limited use because it does not significantly decrease the need to resend data that is corrupted. For example, a noise pulse 0.1 seconds in duration will destroy or impair 480 bits. Consequently, such FEC techniques have not found favor in high speed data transmission systems operating over even moderately hostile links. Further information on FEC and Golay encoding may be found in the following: The Theory of Error--Correcting Codes by F. J. MacWilliams and N. J. A. Sloane, Amsterdam: North Holland, 1977; Analog Transmission Performance on the Switched Telecommunications Network by F. P. Duffy and J. W. Thatcher, Jr. Bell System Technical Journal, Volume 50, pgs. 1311-1347, April 1971; and Data Communications, Networks and Systems by T. Bartec, Macmillan Company, 1985.
In a data transmission system, an uncorrectable, defective bit packet received by the receiver requires that a replacement bit packet be sent by the transmitter. In most high speed systems, the transmitter sends a continuous sequence of packets. The receipt of a bad packet, and subsequent notification thereof by the receiver, causes the transmission to be aborted, with the transmitter having to resend the bad packet and all succeeding packets. This type of operation also significantly degrades system throughput.
The present invention is specifically directed to a high speed, error-free method of data transmission in a very noisy environment, such as a cellular telephone link, but is also beneficial in land-based wire line or other transmission systems. The preferred embodiment of the invention is used with a modem that meets CCITT (Consultant Committee on International Telephone and Telegraphy) Specification V.32. The V.32 specification is currently directed to data transmission rates of 9600/4800 Baud. The inventive method is embodied in a protocol that incorporates a number of novel features including adaptive data compression, selective FEC, interleaving to expand FEC effectiveness, error protection and a data carousel for permitting retransmission of defective digital information packets by interspersing them into the normal sequence of packet transmissions. Each of the aspects of the invention is useful apart from its combination in the preferred embodiment of the invention. The preferred method also uses variable packet sizes, where the size of the bit packets is based, in part, on the rate of data input to increase data transmission. An added benefit of the inventive method is that the transmitted digital information is secure because the processing also encrypts the data.